THERMODYNAMIC AND MECHANISTIC STUDIES OF INITIAL-STAGES IN PROPANE AROMATIZATION OVER GA-MODIFIED H-ZSM-5 CATALYSTS

C-13 MAS NMR has been performed in situ to investigate the early stages in the conversion of propane to aromatics on Ga-containing ZSM-5 catalysts. Propane 2-C-13 was used as labelled reactant. The scrambling of the C-13 label in the very early stages of the propane conversion, even at 573 K, indicates that the first reaction intermediate is a protonated pseudocyclopropane (PPCP) species formed by activation of propane on a (Ga3+,O2-) ion pair and its protonation by a nearby Bronsted acidic site. This PPCP species can decompose in several ways leading to H-2, CH4, C2H4, C2H6, and C3H6 as primary products. The very same molecules can also be produced as secondary products by cracking and hydrogen transfer at high conversion. CH3+, C2H5+ and C3H7+ carbenium ions which are formed by decomposition of PPCP can react further with alkane or olefinic species. Reaction of CH3+ (stabilised by a basic anionic framework oxygen) with propane (activated on a Ga site) may yield n-butane as indicated by the increase in the n-butane/i-butane ratio when the catalyst contains gallium.